1. Field of the Invention
This invention relates to an omnidirectional antenna having almost uniform radiation directivity to a horizontal surface for use with communication systems such as mobile communication, indoor radio, or LAN (Local Area Network).
2. Description of the Related Art
An antenna used for with a radio base station for a mobile communication in the underground shopping center or an indoor base station for mobile radio should be small, thin and be produced at a low cost. In addition, the antenna is required to have such radiation directivity as omnidirectional to the horizontal surface, and as beam-tilted downward to the vertical surface, because it is installed in the underground shopping center or on the indoor ceiling portion.
Conventionally, in the radio base station for mobile communication, in particular, in the base station of a car telephone, a Collinia array antenna arranged multistage dipole elements in the longitudinal direction to realize a beam-tilt directivity. However, it is difficult to use a large-sized array antenna indoors.
As an antenna to satisfy such requirement, "Beam-tilt Dipole Antenna for Indoor Base Station" reported in the great autumn meeting SB-1-8, IEICE of Japan, 1994 is known. This beam-tilt dipole antenna is arranged a plurality of passive elements tilted at a predetermined angle in a form of spatially independent around exciting elements. However, the beam-tilt dipole antenna has a problem of complex structure. In addition, antenna elements are composed of linear elements. Although unit cost of elements is lower, installation requires much labor, thereby raising the production cost. Moreover, when forming an array antenna with the beam-tilt dipole antenna, not only a construction becomes more complicated, but also novel is needed to feed by a coaxial feed line to a number of elements. Thereby, production is difficult and a production cost is raised, as well as an excessive radiation is occurred from a feed line, deteriorating an antenna gain.
Further, in Japanese Patent Published application Ser. No. HEI 6-66578, "omnidirectional microstrip antenna" is disclosed that cylinder passive elements are fed by microstrip feed lines. The antenna alone realizes omnidirection in a horizontal surface, and is compactly constructed easily. When forming an array antenna with the antenna, cylinder passive elements are disposed along with longer sides at predetermined intervals. These elements are fed parallel by the microstrip feed lines. However, in this construction, a plurality of microstrip lines are disposed within a cylinder which forms passive elements to feed a plurality of passive elements. Thereby, not only the construction becomes complicated, but also a radius of the column should be large. Thus, an advantage of reducing a size is lost. Further, in the microstrip feed lines, inner portions of the cylinder passive elements work as antenna elements. Connecting portions of these antenna elements produce an excessive radiation, thereby deteriorating an antenna gain.